The present invention relates generally to an apparatus for dispensing viscous fluids and, more particularly, to an apparatus and method for supplying hot melt adhesives to a dispensing gun.
The ability to precisely dispense viscous industrial materials, such as hot melt adhesives, is a necessity for manufacturers engaged in the packaging and plastics industries. Inconsistent application of adhesive onto a substrate translates into unusable and scrap product and increased costs. Therefore, the process of supplying adhesive to a fluid dispensing applicator or gun must be precisely controlled.
A typical fluid dispensing operation employs a dispensing gun to apply a fluid, for example, an adhesive, onto a substrate being moved past the dispensing gun by a conveyor. The speed of the conveyor, or line speed, is set according to such factors as the complexity of the dispensing pattern and the configuration of the gun. Fluid adhesive is normally supplied to the dispensing gun by flexible hoses. Adhesive is pumped from a reservoir by a metering pump, for example, a motor-driven positive displacement pump. A metering pump for purposes herein is a pump in which the output volume is directly proportional to the action or displacement of the pump independent of fluid viscosity, except for any fluid leakage within the pump. Therefore, with a metering pump, the flow rate of the adhesive being dispensed from the gun is a function of the speed of the motor driving the pump.
The proper application of fluid or adhesive onto a substrate requires that the flowrate of the fluid from the dispensing gun remain as constant as possible throughout the fluid dispensing process. Variations in the flowrate result in different quantities or volumes of fluid being applied at different locations across the substrate. Thus, with too little adhesive, a desired coating thickness is not achieved, and the quality of the adhesive capability is reduced. Similarly, with an excessive quantity of fluid being dispensed, the adhesive may subsequently be displaced to areas of the substrate where it is not wanted; and again, the quality of the substrate product is reduced. In either event scrap product is often the result.
In many applications, the speed of the conveyor carrying the substrate is controllable and changed in accordance with the production line""s capability to produce a high quality product. For example, with a first time run of a product, a production line may be operated at a slower speed to ensure a high quality product. But over time, as the production line is tuned, it can operate at a higher conveyor speed and still produce a high quality product. Assume the fluid dispensing system is operating properly with the conveyor operating at a first constant speed. If the speed of the conveyor and the substrate is increased to a higher constant speed, the flowrate of fluid being dispensed through the gun must also be increased in order to maintain a consistent, high quality coating of fluid on the substrate. It is known to use a signal related to the conveyor speed to modify the speed of the pump motor. Hence, when the conveyor is adjusted to the higher constant speed, the speed of the pump motor increases; and the flow of fluid to the gun is increased, thereby causing the pressure within the gun to increase. The increased gun pressure causes the flowrate of fluid from the gun to increase, and thus, the flowrate of the fluid being dispensed is changed as a function of conveyor speed.
The above flow control system works relatively well while the conveyor is operating at a constant speed, however, the flow control system does not operate properly during periods when the conveyor is accelerating or decelerating. Such conveyor speed changes occur, for example, when the conveyor is initially started from rest. Known systems are unable to maintain the desired flowrate of the fluid through the dispensing gun during periods of conveyor acceleration and deceleration.
FIG. 5A illustrates how the fluid pressure at the dispensing gun changes with respect to an acceleration and deceleration of the conveyor. When the conveyor is at a zero speed (500), with some systems, for example, those using a pressure relief recirculation valve, the recirculation pressure is higher (502) than a desired operating pressure (504) of the dispensing gun. Therefore, when the conveyor line is initially started (506) and is accelerating, the fluid dispensing occurs at an excessive pressure, thereby depositing excessive fluid and producing scrap product. The production of scrap product will continue as the pressure decreases (508) and the conveyor accelerates until both the conveyor speed and operating pressure reach their desired values (509). For purposes of illustration, the desired values of conveyor speed and operating pressure are shown as the common line (504). Upon being given a deceleration command (530), the conveyor speed decreases (532) to a zero velocity (534). However, upon the dispensing gun closing, the pressure rises (536) until the pressure relief valve opens and stabilizes the pressure (538).
In other recirculation systems, a solenoid actuated pressure relief valve is in series with a restricted orifice; and upon the recirculation valve opening, the recirculation pressure (510) is held at a level lower than desired operating pressure. Upon the conveyor accelerating (506), the gun pressure initially drops to a still lower pressure (512) faster than the metering pump can increase the pressure. Therefore, for a short period of time after the conveyor line starts, an excessive amount of fluid is dispensed which results in the production of scrap product. As the conveyor line accelerates, at some point (514), for a current conveyor speed, the correct amount of fluid is being dispensed; but continued conveyor line acceleration (516) with lower pressure (518) results in less than the desired flowrate of fluid through the dispensing gun. Thus, scrap product continues to be produced until the conveyor speed and operating pressure both reach their desired values (504). Upon the conveyor starting a deceleration, the recirculation valve is opened and the pressure decreases until it is stabilized at a value (542) determined by the restricted orifice.
As can be seen in FIG. 5A, with the lower recirculation pressure just described, the conveyor accelerates to its desired speed well before the dispensing gun pressure reaches its desired operating pressure. A significant contributing factor to this extended pressure recovery time is the use of flexible hoses connecting the pump with the dispensing gun. At the desired operating pressure, the hoses expand slightly; and the quantity of fluid being dispensed is small relative to the volume of the hoses. In fact, many times, the quantity of fluid dispensed is no more, and often less, than the expansion, or increased volume, of the hose at the desired operating pressure. Therefore, it takes longer for the pump to restore the desired gun pressure because the pumped fluid has to again expand the hose with fluid in order to achieve the desired operating pressure. As will be appreciated, the graphical representations of the pressure and line speed in FIG. 5 are only exemplary. The acceleration and deceleration of the conveyor often varies nonlinearly and normally is not linear as shown. Further, the acceleration and deceleration of the conveyor may differ from day to day and may be different with different systems. Further, the the exact profile of pressure with respect to time often varies substantially on an instantaneous basis and is not in any respect related to the conveyor speed.
Therefore, there is a need for a fluid dispensing system which maintains a desired flowrate of fluid through the dispensing gun while the speed of the conveyor carrying the substrate is changing, for example, when the conveyor is accelerating from rest to its desired conveying speed.
The fluid dispensing system of the present invention addresses the above and other problems associated with known systems in providing a system for pumping a fluid to a dispensing gun. The fluid dispensing system of the present invention minimizes the production of scrap product during periods of changing conveyor speed. The fluid dispensing system of the present invention is especially useful at the beginning of a production run when the conveyor is accelerating from rest to a desired full production speed. In addition, the fluid dispensing system provides the same benefits at the end of a production run when the conveyor is decelerating from its full production speed to rest. Thus, by reducing scrap production, the fluid dispensing system of the present invention reduces scrap product, maintenance, and the product unit cost.
In accordance with the principles of the present invention and the described embodiments, the invention in one embodiment provides an apparatus for controlling a speed of a motor of a metering pump providing pressurized fluid at a dispensing gun. The dispensing gun is opened and closed to dispense fluid onto a substrate being carried by a conveyor past the dispensing gun. The apparatus has a pressure control producing first motor speed signals as a function of changing speeds of the conveyor and changing pressures of the fluid in the dispensing gun when the dispensing gun is open. A flow control produces second motor speed signals as a function of the changing speeds of the conveyor. A motor control responds automatically to the first and second motor speed signals to produce speed command signals for the motor. The speed command signals operate the motor at speeds causing the pump to provide fluid to the dispensing gun at pressures changing at a rate tracking a rate of change of the speed of the conveyor.
The first motor speed signal from the pressure control operates the pump motor in response to both conveyor speed and fluid pressure at the dispensing gun during an acceleration or deceleration of the conveyor. Thus, the pressure at the dispensing gun changes at a rate that follows the acceleration and deceleration of the conveyor, and the flow of fluid from the dispenser also follows the acceleration and deceleration of the conveyor to dispense the proper amount of fluid on the substrate. When the conveyor reaches a constant full speed, the motor control provides the second motor speed signal to the pump motor, thereby controlling flow of the fluid in accordance with the constant full conveyor speed.
In another embodiment, the invention includes a method of providing fluid under pressure to a dispensing gun with a metering pump connected to a motor. The dispensing gun opened and closed to dispense fluid onto a substrate being carried by a conveyor past the dispensing gun. First, a speed of the conveyor is changed. Then, fluid pressures at the dispensing gun are detected while the speed of the conveyor is changing and the dispensing gun is dispensing fluid. In addition, speeds of the conveyor are detected while the speed of the conveyor is changing. In response to detecting the pressures and the speeds, the fluid pressures at the dispensing gun are changed at a rate substantially tracking a rate of change of the speed of the conveyor. Thereafter, the flow of the fluid is automatically controlled as a function of detecting a full speed of the conveyor.
In one aspect of the invention, first motor speed signals are generated in response to the detected fluid pressures and conveyor speeds, and a second motor speed signal is generated in response to detecting a full conveyor speeds. The control of motor speed is automatically switched from the first motor speed signals to the second motor speed signal in response to conveyor having the full conveyor speed.
In a further aspect of the invention, control of the motor speed is gradually switched from the first motor speed signals to the second motor speed signal utilizing differing proportions of the first and second motor speed signals.